The present invention relates generally to improved structures and methods for overcoming stiction forces in MEMS (Micro-Electromechanical Systems) devices. More particularly, the invention relates to structures and methods for reducing the amount of force required to overcome the stiction forces in a MEMS device that tend to cause a movable plate to stick to a stationary plate in the MEMS device, in order to allow the movable plate to be more easily pulled away from the stationary plate.
A “gap-closing” MEMS device generally includes two conducting parallel plates, where one plate (e.g., the bottom plate) is fixed to the substrate and the other plate (e.g., the top plate) is suspended by a spring or elastic suspension element and is free to move. Generally, an electrostatic drive electrode is located underneath the top plate, providing electrostatic attraction force in response to an applied DC drive voltage in order to pull the top plate down toward the bottom plate. A solid dielectric typically is deposited on the bottom plate to prevent the bottom plate from being electrically shorted to the top plate. In the case of a MEMS varactor, there also is an RF “signal electrode” plate underneath the top plate which may have a high-voltage RF signal applied to it. When the top plate and bottom plate are drawn together such that only the solid dielectric separates them, the varactor capacitance between the top plate and the RF electrode plate has its maximum value, and when the top plate is lifted away from the bottom plate, the varactor capacitance is reduced.
FIG. 1 shows the basic structure of a conventional MEMS varactor 1 in which a conductive bottom plate 4 is disposed on an insulating substrate 2. In FIG. 1, substrate 2 would typically be an insulating layer on which the MEMS device is fabricated. A conductive upper or top plate 8 and conductive plate 4 form the electrostatic “pull-in” mechanism that pulls top plate 8 down toward bottom plate 4. Top plate 8 and the conductive co-planar waveguide section labeled CPW form the varactor. A dielectric layer typically is formed on bottom plate 4. The conductive, movable top plate 8 is supported at each end above conductive substrate 2 and bottom plate 4 by means of suitable spring or elastic flexure structures 10A and 10B so that top plate 8 is parallel to both bottom plate 4 and co-planar waveguide CPW. The varactor capacitance is a function of the distance or gap between the upper surface of coplanar waveguide CPW and the lower surface of top plate 8. A suitable DC drive voltage can be applied between bottom plate 4 and top plate 8 so that the resulting electrostatic attraction between conductive plates 4 and 8 overcomes the force of spring or flexure structures 10A and 10B and reduces the gap between them, thereby increasing the varactor capacitance.
It should be understood that an RF signal applied between top plate 8 and co-planar waveguide CPW also produces an RF electrostatic attraction force. That RF electrostatic attraction force combines with the above-mentioned stiction forces and increases the net force holding top plate 8 tightly against the bottom plate 4. It should be understood that because top plate 8 and co-planar waveguide CPW will attract to each other with either a positive or negative voltage difference between them, the MEMS device in effect creates a rectifying action on an applied AC signal such that there is, in effect, an average DC voltage pulling top plate 8 and co-planar waveguide CPW together even though the electric field between them is switching between positive and negative polarities. This is a problem when upper plate 8 is being held by stiction forces against dielectric layer 4A (or conductive electrode 4) and then the DC drive voltage between upper plate 8 and lower electrode 4 is removed in an attempt to decrease the varactor capacitance by releasing upper plate 8 from the stiction forces and raising it relative to conductive substrate 2. (Stiction forces for MEMS devices are well known and are mainly caused by Van der Waals forces, dielectric charging, and other lesser effects. The stiction is a force that holds two metal plates together once they are touching, even if there is an insulating layer between them.)
When a strong electrostatic force pulls top plate 8 down to drive electrode 4, relatively large stiction forces occur between top plate 8 and the drive electrode 4 and therefore tend to prevent top plate 8 from returning to its upper location when the DC drive voltage is removed. When the DC drive voltage is removed, the upward restoring force exerted by springs 10A and 10B attempts to pull top plate 8 back up but typically the restoring force is unable to overcome the stiction forces. It should be understood that MEMS devices generally tend to have substantial problems with stiction forces, and the restoring force of the spring or flexure devices 10A,B in FIG. 1 typically are insufficient to overcome the large stiction forces and often it is very difficult to instantaneously pull or separate the entire top plate 8 away from bottom plate 4. As a practical matter, the amount of restoring force required can be substantially more than can be provided by the spring or flexure restoring devices 10A and 10B. Furthermore, the greater the restoring force provided by spring or flexure restoring devices 10A and 10B, the larger the DC drive voltage applied between top plate 8 and bottom plate or drive electrode 4 must be in order to pull upper plate 8 down into contact with driving electrode 4. Typically, the required DC voltage is the range of roughly 30-40 volts.
Ideally, top plate 8 would spring back up to its starting position when the DC drive voltage applied between top plate 8 and drive electrode 4 is removed, but unfortunately what often actually happens is that the stiction forces are too large to allow that to happen.
There is an unmet need for an improved structure and method for reducing the amount of force required to overcome stiction forces causing one MEMS plate to stick to another, to thereby reduce the amount of force required to separate the two MEMS plates.
There also is an unmet need for an improved structure and method for reducing the amount of force required to overcome stiction forces causing one MEMS plate in a MEMS varactor to stick to another MEMS plate.